I have seen multiple spellings for Porta Potti, a brand of portable toilets. (Some include Porta Pottie and Porta Potty.) The Porta Potty spelling gets the most hits on Google (about 310,00), followed by Porta Potti (about 288,000). The Porta Pottie spelling falls well behind with just over 30,000 hits.
As with most trademarked items, I try to use the generic name whenever possible. It never fails, however, to have a person use a trademarked name in a quote. Then, I rely on the International Trademark Association, a handy reference for these types of names.